1. Field of the Invention
The present invention relates to a rare-earth magnet and a method for producing the magnet.
2. Description of the Related Art
An R—Fe—B based rare-earth sintered magnet is known as a magnet with the highest performance among various types of permanent magnets, and has been used extensively in a voice coil motor (VCM) for a hard disk drive and in a magnetic circuit for a magnetic resistance imaging (MRI), for example.
In the prior art, an R—Fe—B based sintered magnet embedded in a magnetic circuit has a weight of about 100 g to about 1 kg each. Depending on the applications, some big magnets may have a weight exceeding 1 kg. Recently, however, small-sized sintered magnets have been used more and more often in optical pickups and motors of very small sizes, for example. Some of those small-sized sintered magnets may have a weight of less than 1 g.
A magnetic circuit that adopts such small-sized and lightweight magnets needs to keep the performance of a final product high enough while meeting the demands for reducing the size and weight of the final product. That is why magnets for use in such a magnetic circuit should exhibit strong magnetic properties even though their size is small. Therefore, there have been growing demands for high-performance R—Fe—B based sintered magnets in the field of small-sized magnets, too.
It is known that the coercivity of an Nd—Fe—B based magnet is produced by its internal nanostructure in which an Nd2Fe14B main phase is surrounded with thin Nd-rich phases, thus realizing a high maximum energy product.
However, when an Nd—Fe—B based sintered magnet is actually used in motors, for example, the magnet is usually subjected to a grinding process to finish it in a desired final size and to obtain a predetermined degree of concentricity. During that process, the Nd-rich phase on the surface layer of the magnet is often damaged due to very small grinding cracks or oxidation. As a result, the magnetic properties on the surface portion of the magnet may decrease to only a fraction of the properties inside the magnet.
This phenomenon is observed particularly noticeably in small-sized magnets with a large surface area to volume ratio. For example, if a block magnet of 10 mm square, having a (BH)max of 360 kJ/m3, is cut into rectangular parallelepiped shapes with dimensions of 1 mm×1 mm×2 mm and then subjected to a grinding process, then their (BH)max will decrease to about 240 kJ/m3. As a result, the essential properties of the Nd—Fe—B based rare-earth magnet are not realized anymore.
Also, as a result of the machining process, a machine-degraded layer having no coercivity anymore is always formed on the surface of the sintered magnet. Since the coercivity of the magnet has been lost from such a machine-degraded layer, that layer will not function as a magnet even when magnetized. If the sintered magnet has a sufficiently large volume, such a machine-degraded layer, if ever, accounts for just a small volume percentage. Thus, the overall performance of the magnet is hardly affected by the machine-degraded layer. However, if the sintered magnet has a decreased volume, then the machine-degraded layer accounts for an increased volume percentage. In that case, its influence is non-negligible.
Suppose the volume of the machined sintered magnet is V, the total surface area of the sintered magnet is S, and the thickness of the machine-degraded layer is d. In that case, the volume of the machine-degraded layer is approximated at Sd. Therefore, the volume percentage of the machine-degraded layer to the overall sintered magnet with the volume V is Sd/V. The volume of the intact portion of the sintered magnet, still maintaining coercivity, is given by V-Sd. Thus, the remanence of the overall machined magnet is obtained by multiplying its original value (i.e., before the machining process) by (V−Sd)/V=1−Sd/V. That is to say, the Sd/V value of a magnet becomes an index indicating how much the machine-degraded layer of the magnet affects its magnetic properties.
Sd/V is the product of S/V and d. The former is a factor determined only by the shape of the magnet, whereas the latter is a factor determined by the process history of the machine-degraded layer. The smaller the volume of a magnet, the larger S/V becomes and the smaller d should be.
Hereinafter, conventional techniques of doing some type of processing on the surface of a sintered magnet will be described.
Patent Document No. 1 discloses a permanent magnet material, in which a rare-earth metal such as Nd, Pr, Dy, Ho, Tb, La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu or Y has been deposited on the surface to be machined and turned into a reformed layer through a diffusion process.
Patent Documents Nos. 2 and 3 teach forming a film of titanium metal or a titanium compound such as titanium, a titanium nitride, a titanium carbide or a titanium oxide on the surface of a rare-earth-iron based magnet.
Patent Document No. 4 proposes providing a coating of a compound including Ti and at least one element selected from the group consisting of Nd, Fe, B and O.
Patent Document No. 5 teaches forming a thin-film layer, consisting essentially of Sm and Co, on the ground surface of an Nd—Fe—B based sintered magnet that has been subjected to a grinding process.
Patent Document No. 6 teaches coating the surface of a machined magnet with a refractory metal (which may be Ta with a particle size of 100 μm or less according to a working example) and also proposes embedding the magnet in particles of the refractory metal and dissolving them at a temperature of 700° C. to 900° C.
Patent Document No. 7 discloses a method of improving the loop squareness by depositing Pd or a Pd metal layer on the surface of a machined magnet by an evaporation process, for example, and then melting the machine-degraded layer with a laser beam. Pd is used to get the plating process done more easily.
Patent Document No. 8 discloses a rare-earth magnet that has been machined so as to have an S/V value of 2 mm−1 or more and a volume of 100 mm3 or less. According to Patent Document No. 8, to reform a degraded and damaged portion formed by a machining process, a rare-earth metal is diffused from the surface of the magnet so as to penetrate deeper than the radius of crystal grains that are exposed on the surface of the magnet.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 62-74048        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 63-9908        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 63-9919        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 63-168009        Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 2001-93715        Patent Document No. 6: Japanese Patent Application Laid-Open Publication No. 2001-196209        Patent Document No. 7: Japanese Patent Application Laid-Open Publication No. 2002-212602        Patent Document No. 8: Japanese Patent Application Laid-Open Publication No. 2004-304038        
Recently, there are growing demands for ultra small magnets. The demands are escalating not just in the fields of optical pickups and ultra small motors but also in the fields of cardiosurgery and neurosurgery as well. In the fields of these cutting-edge medical treatments, a technique for controlling the direction in which a vascular catheter advances at a branching point of a blood vessel by attaching a small high-performance magnet to the end of the catheter and applying a magnetic field from outside of the patient's body has been researched. On the other hand, in a magnetic induction surgical system, it has been proposed that an ultra small magnet be embedded at a particular location of the body and used as a location marker. The ultra small magnets for use in such applications should have a cylindrical shape with a diameter of 0.3 mm and a length of 2 mm, for example. In that case, the S/V value exceeds 10 mm−1. Such a magnet needs to have magnetic properties that are high enough to make the magnet work fine irrespective of its small size.
If the size of a magnet is reduced, however, the performance of the magnet, which would be much higher if the magnet had a big size, may not be exhibited fully.
Patent Document No. 1 proposes coating the machine-degraded layer on the ground surface of a sintered magnet with a thin rare-earth metal layer to make a reformed layer through a diffusion reaction. More specifically, Patent Document No. 1 discloses an experimental example in which a sputtered film is formed on a thin test piece with a length of 20 mm, a width of 5 mm and a thickness of 0.15 mm but achieves a (BH)max of only 200 kJ/m3 at most. Also, the surface is oxidized during the diffusion process by annealing, thus causing inconvenience in the subsequent surface treatment.
Patent Documents Nos. 2, 3 and 4 disclose techniques for increasing the corrosion resistance of a rare-earth-iron based magnet to be corroded easily but are silent about how to repair the degradation caused by machining.
According to the technique disclosed in Patent Document No. 5, Sm, diffusing into the magnet as a result of the heat treatment process, deteriorates the magnetic anisotropy of the Nd2Fe14B phase crystals.
According to the technique disclosed in Patent Document No. 6, as long as heat treatment is carried out, it is difficult to minimize the oxidation at the surface of a rare-earth magnet. Consequently, it is also difficult to recover the properties just as intended.
In the method disclosed in Patent Document No. 7, it is not cost effective to deposit Pd metal or melt the machine-degraded layer with a laser beam.
Patent Document No. 8 reports that by depositing a heavy rare-earth element Dy or Tb by a sputtering process and diffusing the element into the mother phase, not only can the magnetic properties be recovered but also can the coercivity be increased significantly. However, the heat treatment process adopted in this method is not cost effective because it is necessary to control the concentration of oxygen in the atmosphere and the dew point thereof with high precision. Besides, this method lacks mass-productivity because a lot of magnets cannot be processed in a single batch.